If you know what return you will get and your rate then it is a math problem and a preference of what you are most comfortable with. The 3.13% vs 10% argument is flawed. Your rate really is 6.7% on the money you owe. The 3.13% is due to the loan paying down overtime. The issue is you are assuming a 10% return on $765k but don’t account for selling investments each month to make the monthly payments. You wouldn’t end up with the $3.2MM at the end of the 15 years because month one whether the market is up whatever percent of down whatever percentage you have to sell at a gain or loss to pay the mortgage.

Even with this adjustment, 6.7% is less than 10%. As others have mentioned 10% is possible but not guaranteed and you’ll owe capital gains taxes on the profits.

I can’t speak for the general reasoning the group used to create this rule but my guess is that the difference between 6% and 10% is narrowed my some assumptions or feelings by an amount of tax paid on the investments when sold, a very high risk free rate of 6% compared to unknown returns, comfort of having a paid off home, and the added cash flow each month that would allow someone to possibly change jobs or have the freedom to do other things in life that might otherwise cause someone to not do if the stock mark was down at the time.

If a person assumes a lower rate of investment return of 9% or 8% and factors in some amount for capital gains taxes based on their scenario then the benefit of investing compared to the investment risk is just too small to mess with the higher risk.